This invention relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to a cardiac rhythm management system and method for preventing recurrence of atrial tachyarrhythmias.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous lead wire or catheter (referred to as a xe2x80x9cleadxe2x80x9d) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as xe2x80x9ccapturingxe2x80x9d the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn""t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation shock. The shock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is the proper treatment of atrial tachyarrhythmias, such as atrial fibrillation. Atrial fibrillation is a common cardiac arrhythmia which reduces the pumping efficiency of the heart, though not to as great a degree as in ventricular fibrillation. However, this reduced pumping efficiency requires the ventricle to work harder, which is particularly undesirable in sick patients that cannot tolerate additional stresses. As a result of atrial fibrillation, patients must typically limit their activity and exercise.
Although atrial fibrillation, by itself, is usually not life-threatening, prolonged atrial fibrillation may be associated with strokes, which are thought to be caused by blood clots forming in areas of stagnant blood flow. Treating such blood clots requires the use of anticoagulants. Atrial fibrillation may also cause pain, dizziness, and other irritation to the patient. For this reason, atrial fibrillation is typically treated with a low energy defibrillation shock to enable the resumption of normal atrial heart rhythms.
An even more serious problem, however, is the risk that atrial fibrillation may induce irregular ventricular heart rhythms by processes that are yet to be fully understood. Such induced ventricular arrhythmias compromise pumping efficiency even more drastically than atrial arrhythmias and, in some instances, may be life-threatening. Moreover, treating atrial fibrillation by a defibrillation shock may also induce dangerous ventricular arrhythmias. For these and other reasons, there is a need for safe and more effective atrial therapy that prevents the occurrence of atrial tachyarrhythmias, such as atrial fibrillation, thereby avoiding inducing ventricular arrhythmia as the result of the atrial tachyarrhythmia or its treatment.
The present subject matter provides a system and method to address the aforementioned problems. In one embodiment, the present subject matter provides a post-defibrillation shock therapy or post-cardioversion therapy which may prevent or slow the recurrence of the arrhythmia which necessitated the shock. Pacing pulses are delivered at an elevated rate relative to normal intrinsic rates after delivering a defibrillation or cardioversion shock. The pacing pulses delivered at this rate initiate and control the contraction of the heart at a rate that is at or just above the heart""s own intrinsic rate. By controlling and initiating the cardiac contractions the likelihood of the arrhythmia which necessitated the shock returning is reduced.
In one embodiment, the present system provides for post-defibrillation/cardioversion shock pacing pulses to be delivered to the cardiac region having received the shock. The system begins by delivering the pacing pulses at a predetermined time after the defibrillation shock or cardioversion pulse has been delivered. Alternatively, pacing pulses of the present subject matter are delivered after treating the heart with antitachycardia pacing. The pacing pulses are initially delivered at a pacing rate having an initial value. In one embodiment, the initial value is set well above the intrinsic cardiac rate of the patient. This ensures that the pacing rate used by the system will control the rate and the refractory period of the heart. In one embodiment, the pacing rate has an initial value set in the range of 100 to 200 pacing pulses per minute.
The rate of the pacing pulses is decreased from the initial value down to a point where the intrinsic rate of the heart is detected. In one embodiment, the decrease in the pacing rate occurs as a function of delivered pacing pulses. For example, the pacing rate is decreased by a set amount after a set number of cardiac cycles, where the set number of cardiac cycles is a programmable number. In one embodiment, the set number of cardiac cycles paced and the decrease in the pacing rate by the set amount is repeated until at least one intrinsic contraction is detected.
As the pacing rate is being decreased, the system senses for the intrinsic cardiac contraction from the paced chamber. Once the intrinsic contraction is sensed, an intrinsic cardiac rate is determined. In one embodiment, the intrinsic cardiac rate is determined between the paced event and the intrinsic event. Alternatively, the intrinsic cardiac rate is determined between two consecutive intrinsic events (e.g., sensed contractions). The pacing rate is then increased to be above the intrinsic rate, where the pacing rate is then maintained just above the intrinsic rate.
These and other features and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention.